Petroleum is a mineral oil formed by a mixture of organic compounds trapped in very diverse geological formations. Thus, each petroleum deposit holds a particular quality of petroleum, determined by the relative proportions of the different organic compounds of which it is constituted.
These organic compounds are essentially hydrocarbons, including saturated compounds: the n-alkanes, iso-alkanes and cycloalkanes, aromatic compounds, resins or also asphaltenes.
Among the cycloalkanes, the diamondoids, tricyclic terpanes, hopanes, steranes may be mentioned. Among the iso-alkanes, pristane and phytane may be mentioned.
The diamondoids are 3-dimensional polycyclic saturated organic compounds. The diamondoids are presented in the form of a cage and can be substituted, or unsubstituted, by alkyl groups. Among these diamondoids, adamantane (C10H16 compound), diamantane (C14H20) or also triamantane (C18H24) may be mentioned non-limitatively, as well as their homologues comprising at least one alkyl branch.
These diamondoids are therefore natural constituents of petroleum, which is also referred to as “oil” in the remainder of the present description. The diamondoids are commonly found in oils at concentrations greater than 1 ppm.
Because of their unique physico-chemical properties (high thermal stability, high melting and vapour pressure points), there has for several years been growing interest in the diamondoids in a large number of fields (pharmaceuticals industry, medicine, nanotechnology, micro-electronics etc.) including the petroleum sector (G. Ali Mansoori, Advances in Chemical Physics, 136, 207-258, 2007). It is therefore very beneficial to isolate, purify and concentrate these diamondoids for subsequent use of the latter, in particular in the fields which have just been mentioned.
Moreover, due to their high stability in oils, it is beneficial to carry out analyses, both qualitative and quantitative, of these diamondoids in order to have a better understanding of petroleum systems, in particular of biodegraded and cracked oils. Such analyses make it possible in particular to evaluate the geological maturity of an oil field and/or the level of thermal maturity of oils, to distinguish between two oils and/or to characterize mixtures of oils, to evaluate the degree of advancement of biodegradation of the oils, to determine the oil/oil or source rock/oil correlations. This explains the increase in the number of studies carried out aimed at isolating and identifying these diamondoids.
Such qualitative and/or quantitative analyses are in general carried out by gas chromatography (GC) or by gas chromatography coupled with mass spectrometry (GC/MS). As for the isotopic analyses, they are carried out by gas chromatography coupled with isotopic ratio mass spectrometry (GC/irMS).
However, in view of the large variety of compounds forming the oil and the very small quantity of these compounds of interest within an oil, it proves necessary to prepare the sample of oil before carrying out its chromatographic analysis (by GC, GC/MS or GC/irMS) in order to purify, isolate and/or concentrate the specific compounds that are to be studied.
The methods described in the literature for isolating the diamondoids, whether from isolated fractions of iso-alkanes and cycloalkanes, or from an oil, are relatively complex.
Reference can in particular be made to the scientific publication by L. Huang et al. (“A novel method for isolation of diamondoids from crude oils for compound-specific isotope analysis”—Organic Geochemistry—42 (2011) p. 566-571) which describes a method for separating the diamondoids from a sample of crude oil. This method comprises the following successive steps:
a first separation step by liquid chromatography, through a column comprising an activated silica gel, in order to collect a saturated fraction of hydrocarbons comprising the n-alkanes and the cyclic and branched hydrocarbons,
a step of concentration under nitrogen of this saturated fraction of hydrocarbons,
a second separation step of this concentrated saturated fraction of hydrocarbons by liquid chromatography, through a column comprising a molecular sieve (Zeolite ZSM-5 of silicalite type), in order to collect the fraction of cyclic and branched hydrocarbons,
a third step of separation of the diamondoids from the cyclic and branched hydrocarbon fraction, this third step being based on the inclusion, in β-cyclodextrin, of the diamondoids contained in the cyclic and branched hydrocarbon fraction, then the destruction of the cyclodextrin polymer by acid hydrolysis.
The third step of separation described in the publication by L. Huang et al. involves the implementation of the following successive operations:
mixing, under stirring for at least two hours, the cyclic and branched hydrocarbon fraction collected after the second separation step, with β-cyclodextrin in solution in deionized water,
separating, by centrifugation, the precipitate formed during mixing,
washing the precipitate with cyclohexane,
solubilizing the washed precipitate in a dilute HCl solution,
heating (at 80° C. for at least 4 hours) the above solution in order to obtain the β-cyclodextrin acidolysis reaction,
after cooling down the solution, extracting the diamondoids using cyclohexane,
after washing the extracted solution with deionized water, then drying with Na2SO4, the solution containing the diamondoids was concentrated under a nitrogen flow.
This concentrated solution is then analyzed by gas chromatography coupled with mass spectrometry (GC/MS). If the publication of L. Huang et al. reports a separation of the diamondoids from the cyclic and branched hydrocarbon fraction, which is carried out successfully, the fact remains that the increase in the number of operations increases the time necessary to obtain the sought compounds, which necessarily has an impact on obtaining the results of the subsequent analyses to be carried out on these products.
This increase in the number of operations furthermore leads to a very low recovery yield. The latter is typically less than 10% of the initial diamondoids content present in the crude oil
Finally, the increase in the number of operations inevitably increases the risks of the loss and/or contamination of the samples.
Cyclodextrins, which are obtained by the enzymatic degradation of starch, are cyclic oligosaccharides which can comprise from six to eight glucose units. A distinction is thus made between three families of cyclodextrins depending on the number of glucose units forming the oligosaccharide: these families are denoted α, β and γ when they comprise six, seven and eight glucose units respectively.
The cyclodextrins, and more particularly the β-cyclodextrins, are known to form inclusion complexes in aqueous solutions with a large variety of polar and non-polar compounds, including the aromatic hydrocarbons and compounds comprising a heteroatom. The internal cavity of β-cyclodextrin, which is essentially formed by carbon atoms and hydrogen, is fairly hydrophobic whereas the external surfaces are hydrophilic in so far as all the hydroxyl groups of the glucose units are turned towards the outside of the molecule. The internal diameter of the hydrophobic cavity of β-cyclodextrin is comprised between 6.0 and 8.5 Å.
As reported in the publications by G. Ali Mansoori (“Diamondoid molecules”—Advances in Chemical Physics—136 (2007) p. 207-258) and by C. Leggio et al. (“Study on the structure of host-guest supramolecular polymers”—Macromolecules−40 (2007) p. 5899-5906), adamantane, because of its size, is inserted with very good adjustment into the cavity of cyclodextrin and, more particularly, of β-cyclodextrin.
The purpose of the present invention is to provide a method for isolating by liquid chromatography compounds belonging to the family of the diamondoids, whether from a sample of cycloalkanes optionally containing iso-alkanes, or more generally from a sample of a mixture of organic compounds, this method at least partially overcoming the abovementioned drawbacks.
More particularly, the invention relates to a method which makes it possible to isolate diamondoids from such a sample of cycloalkanes optionally containing iso-alkanes, or more generally from a mixture of organic compounds, with a minimum number of operations, without the risk of contamination of the samples and/or the loss of the compounds, and in a reasonable time compatible with carrying out qualitative, quantitative and/or subsequent isotopic analyses.